The polymerisation reaction is usually operated in the presence of the monomer, the optional comonomer(s) and preferably an inert hydrocarbon diluent together with the catalyst system(s). Olefin monomer polymerisation plants comprise at least one reactor and at least one polymer powder degassing unit. The said degassing unit usually comprises degassing vessel(s) and a hydrocarbon recovery unit which itself usually comprises compressor(s), heat exchanger(s), knock-out drum(s) and pump(s); liquid purge stream(s) are also advantageously isolated in the said degassing unit(s). Liquid purge stream(s) may also originate directly from the polymerisation loop itself usually at specific locations where impurities and optionally liquids tend to accumulate. These impurities and liquids tend to affect the reaction design and operations (typically in a restricting manner) and also the degassing design and performance. The steady state concentration of these accumulated hydrocarbons thus needs to be maintained at an acceptable level whilst maximising the overall hydrocarbon (reagent/desired diluent) efficiency of the process. This is typically done through selection of optimised purge streams where the accumulated hydrocarbons concentration in the process is greatest (e.g. degassing section).
For the purpose of the present invention and appended claims, an acceptable level of accumulated hydrocarbons in the reactor is defined as a level above which its absorption on the polymer powder will overload the degassing capabilities of the degassing train; for example, the said concentration will either produce off-specification polymer material or force the operator to stop and/or idle the polymerisation operations.
For example, in a gas phase polymerisation process, a preferred acceptable level of accumulated hydrocarbons in the reactor can be the level for which the dew point of the gas in the reactor is lower than the polymerisation temperature, for example lower than the polymerisation temperature (Tpoly) minus a safety temperature margin (Tsm) [Tdew<(Tpoly−Tsm)]; said Tsm being greater than 2° C., preferably greater than 5° C., or even greater than 10° C.
Said liquid purge stream(s) essentially comprises the inert hydrocarbon together with the optional comonomer(s); additionally, the Applicants have identified that the said liquid purge stream(s) comprise(s) non negligible amounts of “other hydrocarbons” which could be detrimental if recycled to the polymerisation reactor. Said “other hydrocarbons” are produced by side reactions occurring in the course of the polymerisation (e.g. hydrogenation and/or oligomerisation reactions) and/or introduced into the polymerisation reaction as impurities of the (co)monomer(s) and/or inert hydrocarbon(s). The existing process characterise the said liquid purge stream(s) as waste streams, which usually implies its burning in either a flare or an incinerator burner. For example, in June 2008 “HydrocarbonEngineering” (http://www.univation.com/downloads/whitepapers/new_innovations_in_gas_phase_technology3.pdf), Dr. Ian Burdett states “Overall hydrocarbon losses from the system are very small, making recycle integration into olefin cracker unnecessary as well as uneconomical.”. The Applicants have now demonstrated that it is possible to treat the said liquid purge stream(s) in an economical way whilst taking into due consideration the impact on the environment.
Environmental friendly developments are considered as critical for the Applicants. In the course of their continuous technology developments, the Applicants have identified that liquid purges coming from their polymerisation plants, e.g. from the degassing unit(s) and/or the polymerisation reactor loop, should be recovered instead of collected as waste streams and incinerated and/or flared. Attempts were made to recover said liquid purge streams and e.g. recycle it into the cracking plant and/or using it as a fuel (e.g. for steam generation). During the implementation of this development, the Applicants have unfortunately experienced critical efficiency problems during the operation of their hydrocarbon upgrading plant and/or burner.
Whilst not wishing to be bound to this theory, the Applicants found that these efficiency problems were generated by the presence of aluminium containing compounds and/or polymer fines which respectively created the poisoning of the cracking catalysis as well as fouling issues.
The objective of the present invention is therefore to develop a process which is environmentally friendly and which improves a safe recovery of valuable hydrocarbons in an olefin monomer polymerisation plant.
The objective of the present invention is also to develop a process for maintaining the level of accumulated inert in an olefin monomer at an acceptable level whilst treating the liquid purge stream(s) in an environmentally friendly way.
The objective of the present invention is also to develop a process which is environmentally friendly and which improves a safe recovery and/or recycle of valuable hydrocarbons in the course of operation of a petrochemical complex comprising an olefin monomer polymerisation plant and a hydrocarbon upgrading plant whilst ensuring efficient operations of the said upgrading plant.
US2007117939 discloses a process for polymerizing one or more monomers with a catalyst system and a fluorinated hydrocarbon diluent (a hydro fluorocarbon). A fluorinated hydrocarbon containing stream, a polymer product stream and a waste stream can be recovered from the mixture; and at least a portion of the waste stream is sent to a non-flare decomposition system to produce a destructed waste stream, and the destructed waste stream is released to atmosphere. US2007117939 does not recognize the issues encountered with the presence of aluminium containing compounds in the C4+ hydrocarbons (accumulated hydrocarbons).
WO2005030811 discloses a process for polymerizing an olefin and the corresponding comprising the steps of removing at least some of the unreacted olefin and corresponding alkane from the reaction zone, purging at least some of the alkane and recycling any remaining alkane and the unreacted olefin to the reaction zone. It also discloses in its FIG. 2 an integrated petrochemical complex comprising a syngas plant, a methanol plant, a methanol-to-propylene plant, and a polypropylene plant. WO2005030811 does not recognize the issues encountered with the presence of aluminium containing compounds in the C4+ hydrocarbons (accumulated hydrocarbons).
WO2006082007 discloses a process for the polymerization of ethylene or of ethylene with further 1-olefins, in which the ethylene is polymerized in the presence of a catalyst in a gas-phase reactor and reaction gas comprising propane and unpolymerized ethylene is circulated to remove the heat of polymerization, wherein the polymer particles are discharged from the reactor, the polymer particles are separated from the major part of the concomitantly discharged gas and the polymer particles are degassed, the gas is freed of entrained fine particles and is separated from a low-boiling fraction comprising ethylene or from a high boiling fraction containing further 1-olefins or alkanes having from 4 to 12 carbon atoms in a first separation stage, a propane fraction is separated off in a second separation stage and this propane fraction is used for degassing the polymer particles discharged from the reactor. WO2006082007 does not recognize the issues encountered with the presence of aluminium containing compounds in the C4+ hydrocarbons (accumulated hydrocarbons).
Thus, the present invention consists in a petrochemical complex comprising an olefin monomer polymerisation plant comprising at least one polymerisation reactor and an optional degassing section, wherein                the polymerisation reactor comprises process hydrocarbons consisting of the monomer, the optional comonomer(s) and preferably at least one inert hydrocarbon diluent, together with aluminium containing compound(s); and        the polymerisation plant comprises liquid purge stream(s) which comprises aluminium containing compounds and potentially polymer fines, together with accumulated hydrocarbons which are different from the said process hydrocarbons, characterised in that the petrochemical complex also comprises a multipurpose hydrocarbon treatment unit which separates the liquid purge stream(s) into        one stream comprising substantially all of the aluminium containing compounds and potentially polymer fines, and        one stream comprising the accumulated hydrocarbons.        
The present invention also provides a process for the polymerisation of an olefin monomer, said process comprising the polymerisation of the monomer and the optional comonomer(s) preferably in the presence of at least one inert hydrocarbon diluent (the “process hydrocarbons”) and at least one aluminium containing compound, said process comprising at least one liquid purge stream which comprises aluminium containing compounds and potentially polymer fines, together with accumulated hydrocarbons which are different from the said process hydrocarbons,
characterised in that the liquid purge stream is separated into
                one stream comprising substantially all of the aluminium containing compounds and potentially polymer fines, and        one stream comprising the accumulated hydrocarbons.        
The present invention also provides a process for maintaining an acceptable level of accumulated inert hydrocarbons in the polymerisation reactor of an olefin monomer polymerisation plant process, said process comprising the polymerisation of the monomer and the optional comonomer(s) preferably in the presence of at least one inert hydrocarbon diluent (the “process hydrocarbons”) and at least one aluminium containing compound, said process comprising at least one liquid purge stream which comprises aluminium containing compounds and potentially polymer fines, together with accumulated hydrocarbons which are different from the said process hydrocarbons,
characterised in that the liquid purge stream is separated into
                one stream comprising substantially all of the aluminium containing compounds and potentially polymer fines, and        one stream comprising the accumulated hydrocarbons.        
In an alternative embodiment, the present invention consists in a petrochemical complex comprising an olefin monomer polymerisation plant and a hydrocarbon upgrading plant, the said polymerisation plant comprising at least one polymerisation reactor, wherein                the polymerisation reactor comprises process hydrocarbons consisting of the monomer, the optional comonomer(s) and preferably at least one inert hydrocarbon diluent, together with aluminium containing compounds; and        the polymerisation plant comprises at least one liquid purge stream which comprises aluminium containing compounds and potentially polymer fines, together with accumulated hydrocarbons which are different from the said process hydrocarbons,characterised in that the petrochemical complex also comprises an interconnected multipurpose hydrocarbon treatment unit which separates the liquid purge stream into        one stream comprising substantially all of the aluminium containing compounds and potentially polymer fines, and        one stream comprising the accumulated hydrocarbons which are recycled into the hydrocarbon upgrading plant.        
The present invention also provides a process for operating a petrochemical complex comprising an olefin monomer polymerisation plant and a hydrocarbon upgrading plant, said process comprising the polymerisation in the polymerisation plant of the monomer and the optional comonomer(s) preferably in the presence of at least one inert hydrocarbon diluent (the “process hydrocarbons”) and at least one aluminium containing compound, said process comprising at least one liquid purge stream from the polymerisation plant said liquid purge stream comprising aluminium containing compounds and potentially polymer fines, together with accumulated hydrocarbons which are different from the said process hydrocarbons,
characterised in that the liquid purge stream is separated into
                one stream comprising substantially all of the aluminium containing compounds and potentially polymer fines, and        one stream comprising the accumulated hydrocarbons which are recycled into the hydrocarbon upgrading plant.        